1. Field of the Invention
This invention relates to leuco dyes, and, more particularly, to ballasted leuco dyes that are suitable for use in photothermographic imaging systems.
2. Discussion of the Art
Photothermographic imaging materials (i.e., heat-developable photographic materials) that are classified as "dry silver" compositions or emulsions comprise (1) a light-insensitive, reducible silver source, (2) a light-sensitive material that generates elemental silver when irradiated, and (3) a reducing agent for the reducible silver source. The light-sensitive material is generally photographic silver halide, which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks. or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver metal (Ag.degree.) is a catalyst for the reduction of silver ions. A progenitor of the light-sensitive photographic silver halide may be placed into catalytic proximity with the light-insensitive, reducible silver source in a number of different fashions, such as partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049), and other methods that intimately associate the light-sensitive photographic silver halide and the light-insensitive, reducible silver source.
The light-insensitive, reducible silver source. is a material that contains silver ions. The preferred light-insensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as light-insensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the light-sensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image.
As the visible image is produced entirely by silver atoms (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is desirable in order to reduce the cost of raw materials used in the emulsion.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Such dye-forming materials include leuco dyes, which are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced, as shown for example in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681. However, when the reactants and reaction products of photothermographic systems that contain leuco dyes remain in contact after imaging, several problems can result. For example, thermal development often forms turbid and hazy color images because of dye contamination of the reduced metallic silver image in the exposed area of the emulsion. In addition, the resulting prints tend to develop color in unimaged background areas. This "background stain" is caused by slow reaction between the leuco dye and oxidizing agent during storage.
One method of inhibiting background stain is to transfer the color-bearing dye from the photothermographic emulsion to a receptor. However, it is essential that the rate of diffusion of the leuco dye and the color-bearing dye be significantly different to provide good color separation and background stability. One method of dye transfer involves thermal diffusion, that is, heat-induced transfer of a dye by diffusion to an image-receiving layer. This method requires a significant difference in rate of thermal diffusion between the leuco dye and the oxidized leuco dye, i.e. the color-bearing dye. Only the color-bearing dye is transferred; the silver image and unoxidized leuco dye are left behind in the emulsion. In addition, the rate of thermal diffusion of other photographic addenda, such as antihalation dyes, acutance dyes, sensitizing dyes, toners, developers, stabilizers, antifoggants, etc., must be low so that these materials remain behind in the emulsion. All of these addenda can cause stain and reduce image stability. Japanese Kokai No. 59-5239 and U.S. Pat. No. 4,594,307 disclose photothermographic systems that rely on different rates of thermal diffusion for the leuco dye and the color-bearing dye.
A system wherein different components have different rates of thermal diffusion presents several problems. Only a limited number of color-bearing dyes are both diffusible and amenable to being formed into a leuco dye. It is difficult for a dye to diffuse through a binder, even more so through several layers of a photothermographic element. In multiple color systems, different rates of dye formation and migration through multiple layers must be controlled to achieve proper color balance in the final print. Also, prolonged heating to induce the dyes to diffuse may cause fogging.
It would be desirable to provide leuco dyes that not only strongly resist migration, but also exhibit a great diffusion differential between the color-bearing dye and the leuco dye, i.e. the reduced form of the color-bearing dye.